What to wear: Wear a black bodysuit underneath, then make or find your own white armor to layer on top. The devil is in the details, here: you might have to do some work to get that helmet right, but it'll be worth it.
How to act: There's no limit to the evil you'll do. Apprehend nearby civilians, shoot at whoever tries to stop you. And make sure you miss a lot, since Stormtroopers are notoriously bad at aim.